The present invention relates to an electrochemical cell intended for powering implantable medical devices. More particularly, it relates to a liquid electrolyte-type electrochemical cell having a reduced height fillport sealing design.
A variety of different implantable medical devices (IMD) include one or more self-contained power sources that power the various components of the IMD. For example, implantable, programmable drug delivery devices (e.g., used to treat pain, spasticity, and cancer), IMDs useful for therapeutic stimulation of the heart, such as implantable cardioverter-defibrillators, pacemakers, etc., typically incorporate one or more power sources. Numerous other battery-powered implantable medical devices are further available.
Regardless of the exact construction and application, suitable power sources or batteries for IMD's are virtually always electrochemical in nature, commonly referred to as an electrochemical cell. Acceptable electrochemical cells for IMDs typically include a cell enclosure or encasement maintaining various components including an anode, a separator, a cathode, and an electrolyte, and are well known in the art. In general terms, the anode material is typically a lithium metal, or, for rechargeable cells, a lithium ion-containing body. Lithium batteries are generally regarded as acceptable power sources due in part to their high energy density and low self-discharge characteristics relative to other types of batteries. The cathode material is typically metal-based, such as silver vanadium oxide (SVO), manganese dioxide, etc. The electrolyte can also assume a variety of forms such as propylene carbonate containing a dissolved salt such as lithium perchlorate, etc.
As previously described, the various components of the electrochemical cell are maintained within a cell enclosure. Depending upon the particular performance requirements of the electrochemical cell, construction of the anode, or cathode, or other components within the cell enclosure itself can vary. The electrolyte, however, is typically in liquid form, and thus must be dispensed or “charged” into the cell enclosure. As such, the cell enclosure itself is typically comprised of separate case and cover components, with the cover forming a fillport through which the electrolyte can be dispensed. Once a desired volume of electrolyte has been dispensed into the cell enclosure, the fillport is then sealed. In light of the intended bodily implant end-use, it is imperative that a complete, hermetic seal be formed that prevents passage of the electrolyte outwardly from the cell enclosure.
Various fillport sealing designs have been suggested for achieving a requisite hermetic seal. One accepted technique is to weld a separate cap or similar structure over the fillport following electrolyte charging, thereby sealing the fillport opening. Unfortunately, this welding operation may conduct heat to the electrolyte, potentially leading to some electrolyte evaporation. The evaporated electrolyte, in turn, may escape into the weld zone, possibly negatively affecting an integrity of the resulting weld (e.g., pin holes, etc.).
In light of the above weld seal integrity concerns, various other fillport designs have been implemented, whereby a primary seal and a secondary seal are created. In particular, a non-welded, secondary seal is initially formed, followed by a more conventional welded primary seal. With this technique, the secondary seal prevents evaporated electrolyte gases from escaping, and thus from negatively affecting the subsequent weld-type seal.
The prevailing technique for establishing the secondary seal is to press-fit an insert piece within the fillport opening itself or within some other component that is otherwise fluidly connected to the fillport opening. For example, U.S. Pat. No. 4,913,986 describes an electrochemical cell having a metal sleeve welded to (and extending outwardly from) the fillport opening in the cover. Following electrolyte dispensement, a plug is press-fitted or interference-fitted into the fillport opening, thereby establishing a secondary seal. Subsequently, a top of the sleeve is hermetically sealed by welding a metal cap in place. Other references, such as U.S. Pat. No. 5,776,632 to Honegger and U.S. Pat. No. 6,203,937 to Kraska similarly disclose an insert component (in particular, a sphere or ball) press fitted directly into the fillport opening formed by the cell's cover.
In order to consistently provide the requisite press-fit sealing between the insert and cover at the fillport opening, the above-described references inherently require that the cover, and in particular the fillport opening, be precisely machined, and requires subjecting both a top and bottom surface of the cover to a finishing operation. As a result, an overall cost of the electrochemical cell, and in particular the cover component, is greatly increased. Further, where, such as with Honegger, the primary weld is formed directly on the insert piece, the electrolyte is in contact with the insert, possibly affecting the subsequently formed weld due to direct conduction of heat. These concerns have been overcome by providing a separate fill tube component that otherwise receives the secondary sealing insert. For example, U.S. Pat. No. 6,132,896 to Sunderland et al. discloses an electrochemical cell having a separately formed fill tube welded to an interior (or bottom) of the cover about the fillport opening. The fill tube is precisely machined and sized to receive the insert (i.e., a ball) in a press-fitted relationship. The cover is still formed to define a fillport opening (within which a separate cap component is welded following the press-fitting operation); however, the cover need not adhere to rigorous dimensional standards, as the critical feature is between the inner diameter of the fill tube and the ball insert. Because only the fill tube is precisely machined, an overall cost of the electrochemical cell is reduced as compared to the previously described references in which an entirety of the cover must be exactingly formed. An additional feature associated with this fill tube design is providing a void between the press-fitting insert and the cap that is otherwise welded to the cover. In particular, a getter material, typically in the form of hollow glass bubbles, is placed within this void, and provides an auxiliary design feature for indicating or detecting gross leaks in the primary seal.
While highly viable and cost-effective, the evolution of electrochemical cells has raised potential drawbacks. In particular, an entirety of the fill tube extends below the cover (or within the headspace provided within the enclosure). By way of example, a length (or extension into the headspace) of a fill tube associated with available electrochemical cells is on the order of 2.03 mm (0.080 inch). With previous designs, this extension, and thus reduction in available headspace volume, was of little concern due to an overall size of the cell itself. As implantable medical devices continue to become smaller, the electrochemical cells utilized with these devices must also become smaller. Simply stated, future liquid electrolyte-type, electrochemical cells based upon a deep drawn case will need to be reduced in height for various applications. One of the issues associated with shorter cell sizes is utilization of the headspace in the cell to maximize capacity. In general terms, the headspace in these designs is the fixed volume in the cell directly below the cover for the cathode, feedthrough, the fillport components, and the cathode weld attachment. As electrochemical cells get shorter and smaller, this headspace volume remains constant in the cell and occupies a larger percentage of the overall available volume. Unfortunately, the currently employed fill tube assembly design inefficiently occupies a relatively large percentage of the available headspace volume. For a reduced-size, liquid electrolyte-type electrochemical cell, then, these volumetric inefficiencies require a reduction in size of the various active components (e.g., electrode), and thus a reduction in available energy. The energy reduction is undesirable.
Electrochemical cells continue to be highly important tools for powering various devices, and in particular implantable medical devices. To this end, while certain aspects of electrochemical cells, such as selected chemistries, anode or cathode construction, etc., continue to evolve, the accepted technique for effectuating fillport sealing has essentially remained unchanged. While the provision of a non-welded, secondary seal in conjunction with a welded, primary seal is well-accepted, the volumetric inefficiencies associated with current fill tube assembly designs may impede the ability to produce reduced-sized cells with acceptable available energy levels on a cost effective basis. Therefore, a need exists for an electrochemical cell having a low profile fillport sealing design.